1. Field of the Invention
The present invention relates to an oxide semiconductor thin film transistor, and more particularly, to an oxide semiconductor thin film transistor including a source electrode and a drain electrode made of oxide.
2. Description of the Prior Art
In recent years, applications of flat display devices are rapidly developed. Electronics, such as televisions, cell phones, mobiles and refrigerators, are installed with flat display devices. A thin film transistor (TFT) is a kind of semiconductor devices commonly used in the flat display device, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display and an electronic paper (E-paper). The thin film transistor is employed to control voltage and/or current of a pixel of the flat display device for presenting a bright, a dark, or a gray level display effect.
According to different semiconductor materials applied in the thin film transistors, the thin film transistors in current display industries may includes amorphous silicon thin film transistors (a-Si TFTs), poly silicon thin film transistors, and oxide semiconductor thin film transistors. The process flexibility of the oxide semiconductor thin film transistor is better than that of the amorphous silicon thin film transistor, and the electrical mobility of the oxide semiconductor thin film transistor is higher than the electrical mobility of the amorphous silicon thin film transistor. Therefore, the oxide semiconductor thin film transistor is currently the front-runner in the competition of replacing the amorphous silicon thin film transistor in the display industry. The material and electrical properties of the oxide semiconductor layer tend to be influenced easily by the environment condition and the manufacturing process conditions. In the traditional back channel etch (BCE) structure, the oxide semiconductor layer tends to be damaged by follow-up processes such as a forming process of a metal source electrode and a metal drain electrode, and the electrical properties of the oxide semiconductor layer are affected accordingly. Therefore, a coplanar structure is also applied for the oxide semiconductor material in the related industries. In the coplanar structure, the metal source electrode and the metal drain electrode are formed before the process of forming the oxide semiconductor layer to keep the oxide semiconductor layer from being damaged. As shown in FIG. 1, in a conventional oxide semiconductor thin film transistor 100, a gate electrode 120G, a gate insulation layer 130, a metal source electrode 140S and a metal drain electrode 140D are formed sequentially on a substrate 110. An oxide semiconductor layer 150 is formed on the metal source electrode 140S, the metal drain electrode 140D and the gate insulation layer 130, which is uncovered by the metal source electrode 140S and the metal drain electrode 140D and disposed between the metal source electrode 140S and the metal drain electrode 140D, after the forming process of the metal source electrode 140S and the metal drain electrode 140D. A pixel electrode may contact the metal source electrode 140D uncovered by a protection layer 160 to be electrically connected to the metal source electrode 140D. In this coplanar structure, the oxide semiconductor layer 150 may be kept from being influenced by the manufacturing process of the metal source electrode 140S and the metal drain electrode 140D. However, the metal source electrode 140S and the metal drain electrode 140D are generally thicker than the oxide semiconductor layer 150, and edges or taper surfaces of the metal source electrode 140S and the metal drain electrode 140D may be uneven because of the influence of the etching process conditions. The covering condition of the oxide semiconductor layer 150 formed over the edges of the metal source electrode 140S and the metal drain electrode 140D may be bad, and the oxide semiconductor layer 150 may even be cracked over the edges of the metal source electrode 140S and the metal drain electrode 140D. The manufacturing yield and the electrical performance of the oxide semiconductor thin film transistor 100 may be seriously affected accordingly. Additionally, the contact resistance between the oxide semiconductor layer 150 and the metal source electrode 140S and the contact resistance between the oxide semiconductor layer 150 and the metal drain electrode 140D are relatively higher, and the electrical performance of the oxide semiconductor thin film transistor 100 may also be affected accordingly.